Question: 3 people can paint 5 walls in 35 minutes. How many minutes will it take for 6 people to paint 7 walls? Round to the nearest minute.
Answer: We know the following about the number of walls $w$ painted by $p$ people in $t$ minutes at a constant rate $r$ $w = r \cdot t \cdot p$ $\begin{align*}w &= 5\text{ walls}\\ p &= 3\text{ people}\\ t &= 35\text{ minutes}\end{align*}$ Substituting known values and solving for $r$ $r = \dfrac{w}{t \cdot p}= \dfrac{5}{35 \cdot 3} = \dfrac{1}{21}\text{ walls painted per minute per person}$ We can now calculate the amount of time to paint 7 walls with 6 people. $t = \dfrac{w}{r \cdot p} = \dfrac{7}{\dfrac{1}{21} \cdot 6} = \dfrac{7}{\dfrac{2}{7}} = \dfrac{49}{2}\text{ minutes}$ $= 24 \dfrac{1}{2}\text{ minutes}$ Round to the nearest minute: $t = 25\text{ minutes}$